1. Field of the Invention:
The present invention relates to a hydraulic tensioner for imparting an appropriate tension to, for example, a timing belt or chain in a vehicular engine.
2. Description of the Related Art:
For a timing belt or chain which transmits rotation between a crank shaft and a cam shaft in an engine, a hydraulic tensioner is in wide use for suppressing vibrations generated from the timing belt or chain during vehicular running and for maintaining an appropriate tension.
FIG. 5 shows a chain drive in an engine for which a conventional hydraulic tensioner is used. In this figure, a hydraulic tensioner A1 is attached to an engine body on a slack side of a chain A6. The chain A6 is entrained on both a driving sprocket A3 which is rotated by a crank shaft A2 of the engine and a driven sprocket A5 which is fixed onto a cam shaft A4.
In the hydraulic tensioner A1, a plunger A8 projects retractably from the front side of a tensioner body A7. The plunger A8 pushes the back side of a tensioner lever A10 at a position near a free end of the same lever. The tensioner lever A10 is pivotally connected to the engine body through a pivot shaft A9, so that a shoe surface A11 of the tensioner lever A10 comes into sliding contact with the slack side of the chain A6 and thereby imparts a tension to the chain.
In the interior of the tensioner body A7, as shown in FIG. 6, there is formed a plunger receiving hole A12 into which is inserted the plunger A8 in a protrudable and retractable manner. A ball check valve A13 is provided at the bottom of the hole A12.
The plunger A8 has a hollow portion A14 which is open at its end face opposed to the ball check valve A13. A plunger spring A15 is disposed in both the hollow portion A14 and the plunger receiving hole A12. Thus, the plunger spring A15 acts between the tensioner body A7 and the plunger A8 to urge the plunger A8 at all times so that a front end portion of the plunger projects to the exterior of the tensioner body A7.
A high-pressure chamber H is formed by a space which is defined by both the plunger receiving hole A12 and the hollow portion A14 of the plunger A8. The interior of the high-pressure chamber H is always filled with oil which is fed from an oil supply source (not shown) through an oil passage A16 and a ball check valve A13.
In the hydraulic tensioner A1 constructed as above, when an impact force is exerted on the plunger A8 from the chain A6 side through the tensioner lever A10 shown in FIG. 5, the internal oil pressure of the high-pressure chamber H rises and the a check ball A17 in the ball check valve A13 blocks an opening of the oil passage A16 which extends through a ball seat A18, thereby preventing a reverse flow of oil to the oil supply source side.
As a result, the internal pressure of the high-pressure chamber H further increases and leaks to the exterior of the tensioner body A7 through a slight gap formed between the outer peripheral surface of the plunger A8 and the inner peripheral surface of the plunger receiving hole A12. At this time, the impact force acting on the plunger A8 is buffered by the resulting flow resistance of oil.
According to a standard construction of the conventional hydraulic tensioner A1 referred to above, a seating surface A19 of the ball seat in the ball check valve A13 in which the oil passage A16 opens as in FIG. 7 is formed as an outwardly expanded tapered surface having a divergent angle of about 90.degree., and the inside diameter of the oil passage A16 is 2.5 mm, while the outside diameter of the check ball A17 is 3.97 mm.
The check ball A17 having the above outside diameter is made of steel and has a mass of about 0.26 g. When the pressure of oil supplied through the oil passage A16 to the high-pressure chamber H side in FIG. 6 is low and when a load imposed on the plunger A8 from the chain side pulsates with a cycle exceeding 100 Hz, the check ball A17 can no longer follow up the variation in the load due to its inertia, with consequent leakage of oil from the high-pressure chamber H side to the oil passage A16, thus causing the backstop force of the plunger A8 to be deteriorated markedly and intensifying the vibration of the chain.
On the other hand, if the pressure of the oil fed to the high-pressure chamber H is increased, it will become possible to suppress the vibration of the chain during vehicular running, but because of an increase in the tension acting on the chain a beat noise is apt to occur between the driving sprocket and the driven sprocket. For this reason it has so far been difficult to satisfy the performance required from the engine side.